The application of hydroxytyrosol in dairy products

I. Antioxidant Properties of Hydroxytyrosol and Its Compatibility with Dairy Products

Hydroxytyrosol, the main active component of olive oil polyphenols, exhibits extremely strong antioxidant capacity (free radical scavenging ability approximately 10 times that of vitamin E). The ortho-diphenolic hydroxyl groups in its molecular structure delay oxidation by capturing oxygen free radicals, chelating metal ions (e.g., Fe²⁺, Cu²⁺), and inhibiting lipid oxidase activity. In dairy products, its dual water-lipid solubility allows it to act on both the aqueous phase (e.g., yogurt matrix) and lipid phase (e.g., cheese fat). It remains relatively stable under acidic conditions (yogurt pH 4.0–4.5) and high-temperature processing (cheese sterilization), providing feasibility for antioxidant fortification of dairy products.

II. Antioxidant Application and Technical Key Points in Yogurt

Antioxidant Mechanism and Quality Enhancement

Milk fat and proteins in yogurt are susceptible to lipid peroxidation and protein oxidation due to oxygen, light, and microbial metabolism during storage, leading to rancidity, browning, and flavor deterioration. Hydroxytyrosol inhibits oxidation of polyunsaturated fatty acids (e.g., linoleic acid) in milk fat, reduces the formation of oxidation products like malondialdehyde (MDA), and decreases protein carbonylation, maintaining yogurt texture (e.g., viscosity) and taste. Studies show that yogurt with 50–100 mg/kg hydroxytyrosol reduces oxidation indices (e.g., peroxide value) by 30%–50% compared to the control group during 28-day storage at 4°C, without significantly inhibiting the proliferation of lactic acid bacteria (e.g., Lactobacillus bulgaricus, Streptococcus thermophilus).

Formula and Process Optimization

Addition Form: Can be added directly as hydroxytyrosol monomer (purity ≥98%) or olive oil extract (containing hydroxytyrosol and its derivatives). Note that other polyphenols in the extract (e.g., tyrosol, oleuropein) may synergize with hydroxytyrosol for enhanced antioxidant effects.

Compatibility Adjustment: The acidic environment of yogurt may promote ionization of hydroxytyrosol's phenolic hydroxyl groups, enhancing its antioxidant activity. However, the addition amount should be controlled to avoid bitterness (threshold ~80 mg/kg), which can be improved by compounding sweeteners (e.g., fructooligosaccharides) or emulsifiers (e.g., Tween-80).

Process Synergy: Pasteurization (70–85°C, 15–30 seconds) retains 70%–80% of hydroxytyrosol. It is recommended to add after cooling to ~40°C to avoid high-temperature degradation. Combine with encapsulation technology for probiotics (e.g., Bifidobacterium) to reduce oxidative impact on microbial activity.

III. Antioxidant Application and Technical Challenges in Cheese

Antioxidant Effect and Quality Maintenance

In cheese (especially high-fat varieties like Cheddar and blue cheese), fat oxidation during ripening is the main cause of flavor deterioration (e.g., "rancid taste") and texture hardening. Hydroxytyrosol penetrates into the fat globule membrane and casein matrix, inhibiting lipase and oxidase activities to delay free fatty acid oxidation. For example, adding 100–200 mg/kg hydroxytyrosol to Gouda cheese reduces hexanal (a fat oxidation marker) content by 40%–60% after 6 months of ripening, while maintaining protein hydrolysis degree (reflecting ripeness). Additionally, its antioxidant property inhibits secondary oxidation products generated during mold growth on cheese surfaces, extending shelf life.

Technical Difficulties and Solutions

Dispersibility Issue: The complex lipid-water two-phase structure of cheese limits hydroxytyrosol solubility in the aqueous phase (~1 g/L at 25°C), causing uneven distribution due to lipid-phase aggregation. Nano-encapsulation technology (e.g., β-cyclodextrin inclusion) can improve water solubility, enhancing stability by 2–3 times and enabling slow release during cheese ripening for sustained antioxidant effects.

Flavor and Color Impact: High-dose addition (>200 mg/kg) may impart slight astringency. It is recommended to compound with natural antioxidants (e.g., vitamin C, rosemary extract) to reduce single-component usage. Meanwhile, phenolic hydroxyl groups of hydroxytyrosol are prone to oxidative polymerization under alkaline conditions (e.g., ripening pH 5.5–6.5), potentially causing slight browning. Control system pH and store in the dark.

Coordination with Ripening Process: Microbial communities (e.g., propionic acid bacteria, yeasts) during cheese ripening may metabolize hydroxytyrosol, affecting its activity. Selecting starter cultures with weak polyphenol metabolism (e.g., Streptococcus thermophilus ST11) can increase hydroxytyrosol retention from 50% to >70%.

IV. Application Prospects and Safety Considerations

As a natural antioxidant, hydroxytyrosol application in dairy products aligns with the "clean label" trend. The EFSA-certified daily safe intake is 1.5 mg/kg body weight. Calculated by conventional addition levels (50–100 mg/kg for yogurt, 100–200 mg/kg for cheese), the intake from 200 g daily dairy consumption is far below the safety threshold. Future research can focus on developing nano-delivery systems (e.g., liposomes, microcapsules) to further improve stability and bioavailability, while exploring synergies with other functional ingredients (e.g., Omega-3 fatty acids, dietary fiber) to expand dairy applications in functional foods.